Interleukin-6 (IL-6), a multi-functional cytokine, promotes megakaryocyte (Mk) maturation in vitro and augments the platelet count in vivo. The hypothesis underlying this proposal is that IL-6 stimulates Mk and platelet structure and function in a dose-dependent manner. At low concentrations, IL-6 may regulate basal Mk growth; at pharmacologic concentrations, the cytokine may augment Mk cytoplasmic maturation and accelerate platelet production; while at higher pharmacologic doses, IL-6 may induce the production of structurally large and functionally hyper-reactive platelets. Specific questions pertinent to IL-6-stimulated megakaryocytopoiesis will be addressed using novel flow cytometric methods to analyze canine Mks and platelets in vivo and human Mks in vitro: 1) Are Mk ploidy, cytoplasmic volume and platelet-specific components separate developmental programs that can be regulated independently by varying the intensity of Mk stimulation with IL-6? 2) Does IL-6 stimulation of platelet production induce unique anatomical changes (e.g., increased or decreased platelet granule proteins) in Mks that are dependent upon the magnitude of the stimulation? 3) Are there measurable functional differences (P-selectin expression, fibrinogen binding capacity, or survival) between IL-6- stimulated and normal plat elets? 4) If platelets produced in response to IL-6 stimulation exhibit increased sensitivity to activation, will they be hemostatically more effective in vivo than normal platelets? If so, then the functional quality of platelets produced following IL-6 administration may be as important as the quantity. The capability to stimulate platelet production with an exogenous cytokine, together with accurate analytical techniques to measure Mk and platelet structure and function in vivo and in vitro, provide a unique opportunity to observe the linkage between Mk structure and platelet function, and may permit resolution of fundamental questions of the significance of Mk and platelet heterogeneity. These questions are especially germane to clinical practice in view of the recent entrance of IL-6 into clinical trials to augment platelet production.